Tower Shield is only useful in Co-Op Play, if you want to spec as a tank. You can actually aggro entire rooms and block-tank them while your allies hit them without fear of reprisal.

Round/Standard sized shields are a good mix all around. They can usually handle all but the worst threats in their quality-tier contemporary zone, but also still let you parry.

Bucklers are ideal for people who have trained their block skill up considerably, and for people who are good at crowd control. The parry bonus is great and can let you dispatch threads quickly, but you can find yourself overwhelemed and block-broken with bad timing, a state at which you take additional damage (usually resulting in an instant death)